TWO-STAGE PLASMA PROCESS FOR CONVERTING WASTE INTO FUEL GAS AND APPARATUS THEREFOR

Abstract

A two-step gasification process and apparatus for the conversion of solid or liquid organic waste into clean fuel, suitable for use in a gas engine or a gas burner, is described. The waste is fed initially into a primary gasifier, which is a graphite arc furnace. Within the primary gasifier, the organic components of the waste are mixed with a predetermined amount of air, oxygen or steam, and converted into volatiles and soot. The volatiles consist mainly of carbon monoxide and hydrogen, and may include a variety of other hydrocarbons and some fly ash. The gas exiting the primary gasifier first passes through a hot cyclone, where some of the soot and most of the fly ash is collected and returned to the primary gasifier. The remaining soot along with the volatile organic compounds is further treated in a secondary gasifier where the soot and the volatile compounds mix with a high temperature plasma jet and a metered amount of air, oxygen or steam, and are converted into a synthesis gas consisting primarily of carbon monoxide and hydrogen. The synthesis gas is then quenched and cleaned to form a clean fuel gas suitable for use in a gas engine or a gas burner. This offers higher thermal efficiency than conventional technology and produces a cleaner fuel than other known alternatives.

Claims

1. A two-stage plasma process for converting waste having organic and inorganic components into fuel gas, which comprises: (a) in the first stage, vitrifying or melting the inorganic components of the waste and partially gasifying the organic components; and (b) in the second stage, completing the gasification of the organic components so that gas from the first stage of the process entering the secondary gasifier is exposed to a high temperature such as to transform essentially all soot present in the gas to CO and to convert essentially all complex organic molecules to simpler molecules CO, CO.sub.2 and H.sub.2, wherein a dust separation and removal step is provided between the first and second stages of the process.

2. A process according to claim 1, in which the fuel gas produced in the second stage is quenched and cleaned to make it suitable for use in a gas engine or turbine for production of electricity or in a gas burner for production of steam or in chemical synthesis reactions.

3. A process according to claim 1, in which the first stage is carried out in a plasma arc furnace.

4. A process according to claim 1, in which the second stage is carried out in a secondary gasifier using a plasma torch with addition of metered amounts of oxygen, air and/or steam.

5. A process according to claim 3, in which the plasma arc furnace is a refractory lined, enclosed furnace provided with at least one direct current graphite electrode adapted to generate a plasma arc to a bath of liquid inorganic material originating from the waste itself and located at the bottom of the furnace.

6. A process according to claim 5, in which said liquid inorganic material comprises a slag layer which is maintained at a temperature of at least 1500° C.

7. A process according to claim 6, in which said liquid inorganic material further comprises a metal layer also maintained at a temperature of at least 1500° C. and located under the slag layer.

8. A process according to claim 5, in which the waste is introduced into the furnace on top of the liquid inorganic material and the organic component in the waste reacts with air, oxygen and/or steam supplied to the furnace in a predetermined amount adapted to achieve gasification of organic material in the waste into a primary synthesis gas containing CO, H.sub.2, CO.sub.2 and N.sub.2 if the waste contains nitrogen or if air is added to the furnace, and also containing some soot and complex organic molecules.

9. A process according to claim 8, in which the organic material in the waste is so reacted as to form a layer of partially treated waste on top of the slag layer and fresh waste is introduced into the furnace on top of said partially treated waste layer which is maintained at a temperature of between 700 and 800° C. and constitutes a cold top for the fresh waste added to the furnace.

10. A process according to claim 1, in which in the first stage, the organic component in the waste reacts with air, oxygen and/or steam supplied to the furnace to achieve gasification of organic material in the waste into a primary synthesis gas containing CO, H.sub.2, CO.sub.2 and N.sub.2 if the waste contains nitrogen or if air is added to the furnace, and also containing some soot and complex organic molecules, and wherein the primary synthesis gas is subjected to the dust separation and removal step in which dust particles larger than a predetermined size are separated and removed.

11. A process according to claim 10, in which the removed dust particles are recycled to the first stage.

12. A process according to claim 4, in which the secondary gasifier is equipped with a plasma torch fired eductor for exposing the gas from the first stage of the process entering the secondary gasifier to a high temperature.

13. A process according to claim 12, in which the high temperature to which gas from the first stage is exposed in the secondary gasifier is between 900° C. and 1300° C.

14. A process according to claim 13, in which the high temperature is achieved mainly by partial oxidation of the gas from the first stage by injection of predetermined amounts of air, oxygen and/or steam to the eductor, and the plasma torch provides only a small fraction of the energy required for maintaining said high temperature.

15. A process according to claim 12, in which the fuel gas exiting the secondary gasifier is cooled down very rapidly to a temperature below 100° C. so as to freeze the thermodynamic equilibrium of the fuel gas and avoid production of secondary pollutants.

16. A process according to claim 15, in which after cooling, the fuel gas is subjected to a final cleaning operation to remove any remaining contaminants.

17. A process according to claim 1, in which the process is carried out under a negative pressure to preclude exit of toxic fumes or of flammable materials from any unit operations.

18. A process according to claim 1, in which an oxygen starved environment in used in the process to preclude dioxin formation.

19. Apparatus for converting waste having organic and inorganic components into fuel gas, which includes: (a) a primary gasifier comprising a refractory lined, enclosed plasma arc furnace provided with at least one graphite electrode; at least one inlet for feeding waste into the furnace; means for feeding air, oxygen and/or steam in metered amounts into the furnace; and a gas take off port for primary synthesis gas produced in said primary gasifier; said primary gasifier being adapted to maintain layers of molten metal and molten slag at the bottom of the furnace and on top of the molten slag a layer of partially treated waste on top of which fresh waste is fed; and said at least one graphite electrode being adapted to generate a plasma arc to the molten slag present in the furnace during the operation; and (b) a secondary gasifier to which the primary synthesis gas is fed, said secondary gasifier being equipped with a plasma-torch fired eductor adapted to expose the primary synthesis gas entering from the primary gasifier to a high temperature such as to transform essentially any soot present in said primary gas into CO and to convert essentially any complex organic molecule to simpler molecules CO, CO.sub.2 and H.sub.2; means for supplying metered amounts of air, oxygen and/or steam into the eductor; said eductor leading to an insulated chamber; and an outlet being provided in said chamber for the fuel gas resulting from the operation, wherein a dust separator is provided between the primary gasifier and the secondary gasifier.

20. Apparatus according to claim 19, in which in the primary gasifier two graphite electrodes are used creating an arc between one electrode and the slag during the operation, and creating a second arc from the slag to the second electrode.

21. Apparatus according to claim 19, in which the eductor provided in the secondary gasifier is made of a high heat metal alloy or is refractory lined or water cooled, and is equipped with the plasma torch at its inlet.

22. Apparatus according to claim 19, wherein dust particles removed by the dust separator provided between the primary gasifier and the secondary gasifier are recycled to said furnace of the primary gasifier.

23. Apparatus according to claim 19, further comprising a gas quenching and gas cleaning means following the secondary gasifier.

24. Apparatus according to claim 19, further comprising an induced draft fan adapted to operate the apparatus under a negative pressure.

25. A process according to claim 11, in which the first stage is carried out in a plasma arc furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] A preferred, non-limitative embodiment of the invention will now be described with reference to the accompanying drawings, in which:

[0034] FIG. 1 is a diagrammatic representation of a preferred embodiment of the present invention;

[0035] FIG. 2 is an elevation section view of a preferred embodiment of the primary gasifier used within the process and apparatus of the present invention; and

[0036] FIG. 3 is an elevation section view of a preferred embodiment of the secondary gasifier used within the process and apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The process of the present invention can be used to process various types of industrial, hazardous or domestic waste in the form of liquids or solids. The solid wastes can be hospital waste, mixed plastics waste, municipal solid waste, automobile shredder residue or the like. The liquid wastes can be spent solvents, used oils, petroleum sludge, municipal water treatment sludge, de-inking sludge or similar liquids. Normally, the waste will comprise organic and inorganic constituents and in most cases, it will be rich in organic materials. When the waste comprises a combination of solids and liquids, the liquid portion should normally not exceed about 30% by weight of the total.

[0038] As shown in FIG. 1, the waste is first introduced into a primary gasifier (12) which is a plasma furnace. This plasma furnace is normally a refractory lined, enclosed, graphite arc furnace, where the plasma is generated by one or several direct current electrodes forming an electric arc, generally as shown in FIG. 2. The plasma is generated by the electricity 16 flowing through graphite rods to a bath of liquid inorganic material, usually slag originating from the waste itself. This slag is maintained at a temperature of 1500° C. or more. Any metal (i.e. non oxidized inorganic material) present in the waste forms a distinct layer below the slag layer. This metal layer is also maintained at high temperature of 1500° C. or more. When starting the system, the slag can be formed from a previous run or be a common inorganic material such as sand or clay.

[0039] The organic material present in the waste reacts with primary air, oxygen and/or steam 14 that is added to the furnace using lances. This process is called gasification. The net result of the gasification process is the production of a combustible gas called primary synthesis gas 18, containing CO, H.sub.2, CO.sub.2 and N.sub.2 if the waste contains nitrogen or when the gasifier is fed with air, since air contains 21% O.sub.2 and 79% N.sub.2 by volume. The primary synthesis gas also contains soot and some complex organic molecules.

[0040] Gasification occurs as the results of a series of complex chemical reactions that can be simplified as follows:

C+O.sub.2->CO.sub.2 (exothermic)

C+H.sub.2O->CO+H.sub.2 (endothermic)

C+CO.sub.2->2CO (endothermic)

CO+H.sub.2O->CO.sub.2+H.sub.2 (exothermic)

[0041] Some of the reactions are endothermic and some reactions are exothermic. The amount of oxygen, air and/or steam fed to the gasifier can be adjusted to balance the exothermic and endothermic reactions so as to minimize the amount of electric energy required in the furnace. Contrary to dissociation, gasification with metered amounts of oxygen, air and/or steam requires minimal amounts of electrical energy to produce the synthesis gas.

[0042] The slag in the primary gasifier 12 is covered with untreated and partially treated waste, also called a cold top. This cold top serves two purposes. First, since the slag is covered with the relatively cold partially treated waste, the furnace roof and spool are not exposed to the high radiative heat from the slag, reducing heat losses in the furnace and increasing refractory life. Second, the cold top favours the condensation of heavy metals onto the partially treated waste and their subsequent fusion into the slag, The slag 20 is periodically removed from the primary gasifier when required.

[0043] However, due to its relatively cold temperatures (700 to 800° C.), the cold top favours the production of complex organic molecules and soot (carbon) in the primary gasifier 12.

[0044] In order to trap the large soot particles, a dust separator 22 is installed at the gas outlet of the primary gasifier 12. Dust 24 that is removed by the dust separator 22 is normally returned to the primary gasifier 12 for further processing.

[0045] The gas exits the dust separator 22, cleaned of large particulates (generally larger than 10 microns). However, it still contains fine soot particulates and complex organic molecules. A secondary gasifier 26 is used to convert the soot and complex organic molecules to CO, H.sub.2 and CO.sub.2. The secondary gasifier 26 operates using electricity 28 in the form of a plasma torch at a higher temperature than the cold top, namely between 900 and 1300° C. and preferably around 1100° C. At this elevated temperature, the thermodynamic equilibrium between C, CO, CO.sub.2, H.sub.2 and H.sub.2O, favours the formation of CO rather than the formation of C (or soot). Also, at this high temperature, complex organic molecules are converted to simpler molecules CO, CO.sub.2 and H.sub.2. Complex organic molecules such as products of incomplete combustion (PIC) are well known pollutants and could be difficult to burn at lower temperatures. The secondary gasifier 26 ensures that they are converted to the inoffensive CO and H.sub.2 form.

[0046] The secondary gasifier 26 is equipped with a plasma-torch fired eductor as shown in FIG. 3. This eductor ensures that all the gas entering the secondary gasifier 26 is exposed to the high heat and the high intensity radiation of the plasma flame. This ensures essentially complete conversion of all or substantially all the components of the synthesis gas entering the secondary gasifier 26 into simple gaseous molecules of CO, CO.sub.2, H.sub.2 and H.sub.2O.

[0047] Two measures are taken in order to ensure high energy efficiency of the secondary gasifier 26. First, the plasma torch 28 provides the activation energy for the conversion reactions, while small metered amount of secondary oxygen, air and/or steam 30 is added, so that the energy required to increase the gas temperature from 800 to 1100° C. is provided mainly by the partial oxidation of the primary synthesis gas 18. Second, the secondary gasifier 26 chamber is insulated with a material such as ceramic wool, in order to ensure minimal heat loss from the chamber.

[0048] The synthesis gas 32 exiting the secondary gasifier 26 is then cooled by cooling water using a water quench 34. In the water quench, the gas is cooled very rapidly, in a few milliseconds, from 1100° C. to below 100° C. This rapid cooling allows to freeze the thermodynamic equilibrium of the gas and, hence, to avoid the production of secondary pollutants such as dioxins and furans. Dioxins and furans are mainly formed from the recombination of chlorine and carbonated compounds (such as CO and CO.sub.2) in the gas. By cooling the gas quickly, this recombination does not have time to occur. The gas is then subjected to gas cleaning 36 which may be a series of known unit operations that will remove remaining contaminants from the gas such as: fine dust, heavy metals, acid gases (hydrogen chloride and hydrogen sulphide), etc.

[0049] The whole system is kept under a negative pressure by the use of an induced draft fan 38. This ensures that no toxic fumes can exit the system and that the flammable H.sub.2 and CO stay inside the system, limiting the dangers of fires or explosions. The fan can be of turbine or positive displacement type, depending on gas composition. Gas composition will be a function of operating conditions and type of waste being processed.

[0050] The output of the system is clean combustible fuel gas, which can be used for different applications. First, it can be burned in a gas engine or gas turbine 40 for the production of electricity. In that case, cogeneration is also possible: the waste heat from the engine or turbine can he used to produce steam and/or hot water. Depending on system size and waste type, the electricity produced by the engine or turbine may be enough to run the plasma arcs of the primary gasifier 12 and/or the plasma torch of the secondary gasifier 26. The gas can also be used as a source of heat for a boiler 42. In that case, the gas is burned in a standard burner, just as any other commercial gas such as natural gas or liquid petroleum gas (LPG). It can also be used for chemical synthesis 44 as a reaction gas. In all these cases, since the fuel gas has been cleaned essentially of all contaminants, the emissions from the burning or processing of this gas will also be clean of any contaminants.

[0051] FIG. 2 illustrates the preferred embodiment of the primary gasifier 12. The solid and liquid wastes are introduced. into the primary gasifier 12 as a waste mixture through an isolation valve 46 and into one or multiple feed chutes 48. Alternatively, liquid waste may be fed trough an injection nozzle 50 into partially treated waste 52 inside the furnace. By feeding the liquid waste into relatively cold zones of partially treated waste 52, one ensures that the gasification reactions of the liquid waste are progressive, rather than violent and sudden, which would occur if liquid waste were fed directly on top of the hot slag 20.

[0052] The waste is laid over a pool of slag 20 and molten metal 21. The slag and metal are maintained in a liquid state at a temperature of 1500° C. or more by the use of plasma arcs 54 and resistive heating (not shown). The plasma arcs 54 are generated by one or more graphite electrodes 56 that carry DC electric current. Current typically flows from one electrode to the other when more than one electrode 56 is used, creating an arc between one electrode tip 57 and the slag 20, then passing through the highly electrically conductive hot slag 20 and molten metal 21 and creating a second are from the slag 20 to the second electrode tip 57. The electrodes are typically submerged in waste 52, and the plasma arcs 54 are typically covered by waste 52. This favours the passage of current inside the hot slag 20 and molten metal 21, rather than through gas, directly from one electrode to the other. The slag 20 is covered with partially treated waste 52 also referred to as a cold top. Fresh waste 51 is continuously or intermittently added as the gasification reactions in the furnace reduce the volume of waste 52 present.

[0053] Waste 52 is heated by plasma arcs 54, which favour the conversion of the organic components of the waste into CO and H.sub.2. This process is referred to as the gasification reactions. Air, oxygen and/or steam are added through a lance 58, in order to favour the gasification reactions in the highest temperature zones of the primary gasifier 12.

[0054] The inorganic components of the waste melt and form two distinct layers: a bottom layer of the denser metal 21 and a top layer of the lighter slag 20. Once cooled, this slag 20 becomes a glassy rock, which can be used for construction or other purposes. The rock is non-leaching in nature and allows to trap heavy metals and other contaminants into a glass matrix. Slag 20 and metal 21 can he extracted separately from the furnace through two distinct tap holes 60 and 62.

[0055] In the primary gasifier 12, the organic molecules in the waste react with sub-stoichiometric amounts of oxygen, air and/or steam (i.e. less than the oxygen required for complete oxidation of the waste) to form the primary synthesis gas 18. Steam used in the primary gasifier can come from water already present in the waste or be added separately.

[0056] The primary synthesis gas 18 is normally composed of combustible CO, H.sub.2 and of non-combustible CO.sub.2 and N.sub.2. Since the slag is covered by partially treated waste or cold top 52, the gases exit the primary gasifier at a relatively low temperature (800°C). Because of the relatively low temperatures involved in cold top operation, the primary synthesis gas 18 also contains soot and complex organic molecules (such as ethylene, acetylene and aromatic compounds).

[0057] The advantage of cold top operation is higher energy efficiency for two reasons: 1) the furnace spool 64 (top section) is kept at a low temperature and 2) the primary synthesis gas 18 exiting the furnace has a lower temperature.

[0058] By keeping the spool 64 cold, the radiative heat losses to the roof are much reduced. The radiative heat losses are a function of temperature to the 4.sup.th power (q=ϵσ(T.sub.1.sup.4−T.sub.Surr.sup.4)). in consequence, the effect of covering the slag by partially treated waste and reducing its temperature from 1500° C. to 800° C. produces a reduction in radiative heat loss of about 10 times.

[0059] Reducing the temperature of the primary synthesis gas 18 also reduces the sensible heat of the gas exiting the furnace and, therefore, the sensible heat carried out of the furnace.

[0060] Another advantage of the cold top operation is to limit entrainment of particulates. Because the fresh waste 51 falls on a relatively cold surface of the waste 52 being processed, the gasification reactions are less violent and happen in stages as the waste progresses down from cold top temperature to reaction temperature of 1500° C. at the slag 20 surface.

[0061] A still further advantage of cold top operation is to minimize the volatilization of metals, volatilized metals at the high slag temperature condense on the cold waste particles and have a better chance of being trapped in the slag.

[0062] Due to the lower temperatures on the top of the reactor, some waste will exit the reactor unreacted or partially reacted. For example, some oil waste will vaporize before being completely dissociated into CO and H.sub.2. The thermodynamic equilibrium under the reducing conditions of the furnace favour the production of carbon soot at the relatively low temperature at the outlet of the furnace (800° C.). A secondary gasifier 26 working at around 1100° C. is used to convert any remaining complex organics in the primary syngas to CO and H.sub.2. It is shown in FIG. 3 of the drawings. The carbon soot is converted to CO by the addition of oxygen, air and/or steam to the secondary gasifier. At 1100° C., thermodynamic equilibrium, under reducing conditions, favours the production of CO, rather than soot (C).

[0063] The use of the secondary gasifier 26 also gives the option of controlling the chemistry of the fuel gas or secondary synthesis gas 32 produced by the system, without affecting the operation of the primary gasifier 12 (dust entrainment, electrode erosion, slag volatilisation). For example, adding steam into the secondary gasifier 26 will tend to increase the amount of hydrogen present in the secondary synthesis gas 32, while reducing the amount of carbon soot and carbon monoxide.

[0064] The secondary gasifier 26 includes a high temperature chamber 66, equipped with a gas mixer or eductor 68 at the chamber inlet. The inside walls of the eductor 68 can have different construction: refractory-lined, water-cooled, or high heat metal alloy. The eductor is equipped with a plasma torch 70 at the inlet. The eductor 68 provides a suction effect on the primary synthesis gas and favours intimate contact of the soot particles and complex organic molecules with the plasma flame in the eductor throat 69. The high temperature chamber is insulated with insulation 67 in order to ensure minimal heat loss from the chamber.

[0065] The present invention is not limited to the specific embodiments described above, but may comprise various modifications obvious to those skilled in the art without departing from the invention and the scope of the following claims.